Polypropylene (PP) is a commodity polymer that is used in a variety of commercial applications. However, due to its linear chain structure, PP has low melt strength and lacks strain hardening behavior. This limits its application in certain areas that require melt extension. Such application areas include, but are not limited to, thermoforming, blow molding and foaming. The lack of strain hardening behaviour leads to problems including very narrow processing conditions windows, non-uniform foam cell size and pin-holes in sheet thermoforming.
Polypropylene (PP) is one of the most common thermoplastics in the plastics industry with numerous applications ranging from household appliances to automotive interiors. Each application requires a specific PP grade with a specific average molecular weight (MW) and polydispersity index (PDI). PP molecular weight and PDI affect the melt flow behaviour, processing characteristics and eventually the final applications of PP.
However, the use PP is limited in applications requiring significant melt strength. Thus, modifying the molecular structure and enhancing strain hardening of PP melt, can lead to uses in areas such as foaming, thermoforming, extrusion coating and blow moulding.
The high melt strength of a polymer can be attributed to either long chain branching (LCB) or high MW. Hence, attempts have been made over the years to introduce LCB to PP chains. Methods such as electron beam radiation, gamma radiation or utilizing peroxides in the presence of coagents (like styrene or allylic and acrylic multi-functional monomers) have been utilized to impart LCB and increase the number of long chain branches in the PP chains.
Electron Beam Radiation
Electron beam radiation of PP has been used extensively to modify its melt strength. Linear PP pellets are irradiated in a vessel with electrons generated by an electron beam accelerator. Irradiation of PP is carried out under N2 atmosphere to discourage chain scission as much as possible. Then, irradiated PP samples are heated at an elevated temperature to bring the entrapped radicals in the crystalline domains into the interface between crystalline and amorphous regions. This is done to encourage bimolecular termination of the trapped radicals, which leads to formation of long chain branches.
Electron beam (EB) irradiation is proposed in U.S. Pat. Nos. 5,414,027, 5,541,236, 7,169,827, 6,774,156, 7,019,044 and 2,948,666.
Gamma Irradiation
Another technique for rheology modification and increasing of the melt strength of PP is using gamma radiation. In this method, energetic ions and excited states are produced using Cobalt 60 as a gamma ray source. However, good control is lost due to the intensity of gamma radiation. As a result, gamma irradiation can also abstract hydrogens from the PP backbones and cause β-scission.
Gamma-ray irradiation is proposed in U.S. Pat. Nos. 4,916,198, 5,591,785, 5,731,362 and 5,883,151.
Free Radical Initiated Reactions
Peroxide initiators have been used along with co-agents such as triallyl trimesate (TAM), trimethylopropane triacrylate (TM PTA) and triallyl phosphate (TAP) to modify the rheology of PP by introducing LCB. This technique is popular since it is less expensive (and less energy intensive) than the previously described radiation techniques. The effects of allylic and acrylic co-agents on molecular weight and branching distribution were studied by comparing shear and elongation viscosities. It was found that a mix of degraded chains, slightly branched polymer chains and hyper-branched chains, which can only be formed after the gel point, were present in the polymer melt when co-agents were used.
The following mechanism has been suggested for the reaction between PP, peroxide and co-agent. After PP macro-radicals are formed because of the presence of the peroxide initiator, they will attack the carbon double bond on the co-agent, and then a stable radical adduct will be formed. This stable intermediate radical adduct is protected from β-scission. In addition, the hydrogen on this intermediate adduct can react with other degraded polymer chains. These degraded polymer chains are produced from the initial β-scission reaction and contain a terminal double bond. These terminal double bonds react with the intermediate radical adducts, leading to the formation of long chain branches and/or crosslinks. Eventually, the final PP structure will be a function of the yield and the selectivity of the peroxide in the degradation reaction and the co-agent that assists in the crosslinking (CL) step.
Free radical initiated reactions using multi-functional co-monomers is described in U.S. Pat. Nos. 5,047,485 and 5,416,169.
UV Irradiation
Yet another technique that can be employed is UV radiation. UV radiation is a cheaper and safer process for generating free radicals in PP and modifying its molecular structure. In this method, PP is mixed with photoinitiators and UV energy is utilized to activate the photoinitiator. After activation with UV radiation, these initiators can abstract hydrogens from the PP backbone. Hydrogen abstraction will be followed by scission and degradation of PP chains. Degradation of PP using UV energy along with photoinitiators was successful in a twin screw extruder to decrease the polydispersity index (PDI) of PP and produce controlled rheology PP [10]. In addition, in order to control the degradation level of PP in the melt state and form long chain branched PP (LCBPP) for foaming applications, multi-functional acrylic coagents were used along with photoinitiators. The radiation was carried out in the last two zones of a twin screw extruder by using a transparent barrel.
The effect of different photoinitiators and coagents on the amount of gel formed in PP films radiated with UV energy has also been investigated. Moreover, different photoinitiators along with coagents were used to introduce LCB to linear PP and increase its melts strength.
UV irradiation with the use of co-monomer is proposed in U.S Pat. No. 8,703,836.
Extruders have long been used as continuous reactors for polypropylene (PP) chemical modification [74-77]. This process is known as reactive extrusion (REX) and it has been employed to produce controlled rheology PP (CRPP) [1, 78]. In order to produce CRPP via REX, PP and peroxides are fed into the extruder, and initiation reactions followed by β-scission of the PP chains take place during melting and mixing in the extruder. These reactions are responsible for degradation of PP. As soon as temperature reaches the peroxide decomposition temperature, the peroxide abstracts hydrogens from the PP backbones and macroradicals are thus formed. Since PP tertiary radicals are unstable, the chain will break β-scission) and polymer with lower molecular weight (MW) and narrower molecular weight distribution (MWD) will be formed [1].
Utilizing thermo-chemical initiators, such as peroxides, in REX has its own disadvantages, such as limited controllability. Peroxides reach their decomposition temperature prior to effective mixing with PP, thus causing an excessive and non-homogenous degradation in PP. In order to overcome this issue, photoinitiators were used along with UV irradiation to efficiently degrade PP. In this way, the reaction initiation step and subsequent formation of macroradicals become independent of the processing temperature and the reaction only starts when UV irradiates the PP/photoinitiator mixture. He et al. [20] used this technique to modify PP rheology during extrusion. Photomodification was conducted in the last two zones of the extruder by opening the barrel and exposing the mixture to UV irradiation.
Photoinitiators were not only used to produce CRPP, but also to modify the melt strength of PP or other polyolefins by incorporating long chain branches (LCBs) to their structure [5, 79-81]. Increasing PP melt strength is possible by introducing LCBs to the PP structure. Producing long chain branched PP (LCBPP) is more challenging than CRPP, since β-scission reactions should be controlled by stabilizing PP radical centers. This is not trivial, since β-scission reactions are dominant at temperatures above 60° C. The typical temperature for PP processing is well above 60° C. (Tprocess>160° C-PP melting point); thus, excessive degradation is inevitable.
Since LCBPP has numerous commercial applications, there is a need to find a method to continuously modify PP and scale up the system. He et al. [49] irradiated a PP/photoinitiator (BPH)/pentaerythritol triacrylate (PETA) mixture during melt mixing in the extruder by removing the barrel (REX). They found that the modified PP had better foamability due to greater melt strength compared to the parent PP. PETA was used as a crosslinking agent along with BPH to generate long chain branched PP. However, possible drawbacks of UV radiation in the extruder include limited UV penetration depth into the thick plastic melt and excessive degradation due to high processing temperature. As mentioned before, the latter issue makes formation of long chain branches possible only if coagents (like PETA) are used to “block” β-scission reactions.
Despite the various approaches proposed, there remains a need for a solution to produce PP having LCB, safely and easily without the addition of comonomers or coagents.